(USA Today) -- Adam Lanza likely picked Sandy Hook Elementary School for his killing spree because it was the easiest target for an alleged attempt to outdo Norwegian mass murderer who killed 77 people in July 2011, according to two media reports.

The Hartford Courant, quoting unidentified law enforcement sources, reported Tuesday that the theory is based in part on several news articles about Norwegian killer Anders Behring Breivik that investigators found in one of Lanza's two bedrooms at the home he shared with his mother in Newtown, Conn.

The newspaper cautions, however, that sources are treating the Breivik rivalry motive as just one theory.

In December, the 20-year-old Lanza killed his sleeping mother before murdering 20 first-graders and six educators at the school. He then committed suicide as police closed in.

CBS News reported Monday night that two officials briefed on the investigation said Lanza was likely acting out a video game fantasy and saw himself as a competitor to the right-wing fanatic who killed eight people in an Oslo bombing before slaughtering 69 young people on an island camp.

Breivik was sentenced last August to 21 years in prison.

Lanza spent hours alone in a basement gaming room and also went to a shooting range with his mother. Police said he used three weapons in the Sandy Hook massacre, including a semiautomatic assault rifle, and shot his mother with fourth weapon.

Lanza targeted the school because it offered the "largest cluster of people," the sources told CBS.

The Courant also quoted sources on Lanza's movements inside the school that day. One theory is that he likely bypassed the first two classes he encountered and began his assault in a far room so that he would work his way back to the front entrance in order to get more ammunition from his nearby car if needed.

The law enforcement sources also said that Lanza likely was shooting at a teacher standing near a window when bullets hit three car in the parking lot and was not firing at police outside as initially surmised, the newspaper reported.

Police have not officially released a motive for Lanza. Connecticut State Police said that the investigation continues and that any statements about Lanza's intentions were just speculation.